The בני יששכר asks why we do not make a שהחיינו on the מצוה of בדיקת חמץ. He explains that the שהחיינו that we make on the מצוה of תשובה that we made on יום כיפור. The idea, he explains, is that although we may not have lived up to our commitments that we made on יום כיפור, we are reminded again on the eve of בדיקת חמץ to advance those convictions, search out the חמץ and reinvigorate our bond with the רבונו של עולם.